Shutter speed, f-stop and ISO

Plenty of people take wonderful photographs without much understanding of just how the camera settings control light. They understand a few basics like using a fast shutter speed for action, using a tripod for very slow shutterspeeds, boosting ISO in dim light and so on. Lots of folks just set the camera on automatic and concentrate on the subject. Nothing wrong with any of this and if the above describes you it’s okay. But.. if you want to really move to the next level of photography you need to have a pretty firm grasp of the interaction of the three things you can control.

These three settings work together to determine how much light gets through your camera lens. In anything other than full manual mode, if you change one of these then at least one of the others needs to change. Various combinations of these settings let in the same amount of light – of course there are other consequences.

Shutter speed and f-stop are the main players on this stage. Before digital photography they were the only things you could adjust once you loaded your film camera with film of a certain ISO. So let’s look at these two.

Imagine that the amount of light required for a proper exposure of your subject is contained in a cylinder which will pass through your lens. Assume that the camera tells you that f11 and 1/125 are the correct aperture and shutter speeds. That’s a pretty tall cylinder such as that pictured on the left. It will take 1/125th of a second for the cylinder to pass through your lens. Maybe it’s windy and you want a faster shutter speed so you set the camera to 1/250th of a second. You have cut in half the time that the lens will be open to accept all that light. Perhaps you can see that if the cylinder remained tall it would only make it halfway through before the shutter closed. So, the cylinder is going to need to be wider – twice as wide in fact, to get the right amount of light through. So your camera will tell you that f8 is the proper f-stop. (I know, it makes no sense that f8 is twice as wide as f11). If you change the shutter speed to 1/500th the cylinder is going to have to be twice as wide again – f5.6. These two settings then work hand in hand to allow a certain amount of light. If you double one then you must halve the other and vice versa. If you change the aperture (f-stop) the shutter speed will change in direct relation, i.e. changing the f-stop from f4 to f5.6 cuts the lens opening in half and requires a longer (by half) shutter speed.

The third variable is ISO. This number refers to “sensitivity” – how much light does there need to be for a reasonable picture. In the old film days, color film generally was available with an ISO of 25 or 50 or 100 or 400. Black and white film could go as high as ISO 1600. The lower the number the more light needed. Digital cameras provide ISO’s of 50 to 1600 or even 3200 and higher. PIcture the film plate of your camera as a cube that needs to be filled with light.

Maybe you can see where this is going. That first cube, at ISO 100, needs a lot of light. So it is going to require a pretty big cylinder – some combination of relatively wide open f-stop and or slow shutter speed. If you set the camera to ISO 200 you are cutting in half the amount of light that will be needed. If you followed the explanation above about f-stop and shutter speed you’re thinking to yourself “hey I can double the shutter speed or halve the f-stop”. Bingo! That’s exactly how it works. Let’s say you’ve opened up your lens as wide as it can go at f4. The shutter speed the camera wants to use is just too slow, say 1/15th. So you bump the ISO to 200 and presto the camera now wants to use 1/30th. Maybe you’re in a museum and nothing works as ISO 100 or ISO 200. You go on up to ISO 800 or even ISO 1600, cutting that cube pictured above in half with each step. Eventually the camera tells you that it can take a picture at an acceptable shutter speed and f-stop combo.

So then if all these things are equivalent and work together how do you decide which to use? Why not pick a high ISO and middle of the road f-stop and shutter speed and go with it? Well because each of the trio has it’s own cost (feature). Revisiting our first diagram

Shutter speed is perhaps the easiest to understand. For moving subjects you want fast shutter speeds. And if the shutter speed gets too slow (below 1/30th) camera shake can cause blur. The longer the lens (or the more you are zoomed in) the faster the shutter speed needs to be to avoid blur from camera shake. It used to be that a rule of thumb was use a shutter speed at least equal to the length of your lens i.e. with a 250mm telephoto use at least 1/250th. With various vibration reduction technology (known as ISR or VR) built-in to today’s cameras or lenses you can push that a bit though.

The higher the ISO the more “noise” (sometimes thought of as graininess from the film days). So, the lower the ISO the “clearer” your image will be. Some people try to shoot everything at ISO 100. For early digital cameras this was a good idea. Most modern cameras do fine at ISO 400 or ISO 800. If you have a newer DSLR you can get pretty good results even at IS0 1600 so don’t be afraid to crank up that ISO when necessary.

f-stop affects depth of field (DOF). That’s a whole subject for another post but basically DOF has to do with how much of a picture is in focus. At very small f-stops (f22 for example) most everything will be in focus. At very wide (f1.8 for example) DOF will be “shallow” meaning areas of the picture in front of and behind the subject will be out of focus. DOF is much narrower with long lenses (or zoomed out on your compact digital). Macro or close-up photography requires much more attention to DOF than scenics, cityscapes and other “regular” photography. Some of you want to achieve that shallow DOF that you often see in portraits , flower photography, etc. You know the look – the person or flower is sharp but the background is out of focus. This isn’t easy to achieve with the gear most of us have. Reasonably priced lenses only open up to f4 or even smaller. If you buy a “fast” lens with a maximum aperture of 1.8 or 1.4 you can do it but generally with digital cameras and the common lenses you get lots of depth of field. My advice is to concentrate on a sharp subject and don’t worry so much about trying to blur the background. Later you can use your photo editing software to blur the background as much as you want.

Some quick tips that fall out of all this:

With a point and shoot camera stay at IS0 200 and below. With a DSLR feel free to go to ISO 400 and even ISO 800.

For normal everyday shooting set your camera to f8 in aperture priority mode and see if the camera gives you a shutter speed you like. If it doesn’t, open up the lens. If you’re all the way out to as wide as your lens will open and that isn’t working then boost the ISO.

If you’re shooting at a fairly wide open f-stop then concentrate on being sure that the camera focuses on your main subject Sometimes these fancy cameras get fooled and focus on something closer to the camera than your subject. Adjust the autofocus points if your camera offers that or compose the shot with the subject right in the middle, hold the shutter button half way down and then move then camera so that the subject is wherever you want it to be in the frame.

3 Comments

Excellent! One of the camera clubs was having a speaker talk about this topic, and I am not able to go — so this was very much appreciated, and I learned that I’m not too terribly ignorant on this topic.
Thank you, Art!